1. Field of the Invention
The present invention relates to a creasing device that forms a crease (folding crease) on a sheet-like member (hereafter, referred to as a “sheet”) at an intended position before the sheet is folded and an image forming system that includes the creasing device and an image forming apparatus.
2. Description of the Related Art
Conventionally, what is called as saddle-stitched or center-folded booklet production has been performed. The saddle-stitched booklet production is performed by saddle stitching a sheet bundle, which is a stack of a plurality of sheets discharged from an image forming apparatus, and folding the saddle-stitched sheet bundle at a middle portion of the sheet bundle. Folding a sheet bundle including a plurality of sheets causes an outer sheet of the sheet bundle to be stretched at a crease by a greater amount than an inner sheet. An image portion at the crease on the outer sheet may thus be stretched, resulting in damage, such as come off of toner, to the image portion. A similar phenomenon can occur when another kind of folding, such as Z-folding or triple folding, is performed. There is also a case where folding is insufficiently performed due to thickness of a sheet bundle.
There is a well known technology for preventing toner from coming off using a creasing device. The creasing device creases a sheet bundle prior to a folding process where the sheet bundle is folded in two-folding or the like to make an outer sheet easy to be folded. The creasing device typically forms a crease on a sheet in a direction perpendicular to a sheet conveying direction by moving a roller on a sheet, irradiating a laser beam on a sheet, pressing a creasing blade against a sheet, or the like.
A known example of a creasing device is disclosed in Japanese Patent Application Laid-open No. 2009-166928. In Japanese Patent Application Laid-open No. 2009-166928, a technology is disclosed for moving a creasing member by using a plurality of individually-advancing-and-retracting mechanisms, which are activated at different times so as to press a sheet by the creasing member with a gradually-decreasing amount of pressing for producing a crease.
However, producing a crease on a sheet with a roller involves movement of the roller across a length of the sheet in a direction along which the sheet is to be folded, and therefore is time consuming. This can be resolved by rotating the sheet conveying direction by 90 degrees and producing a crease parallel to the sheet conveying direction; however, this scheme involves a change in footprint and therefore is disadvantageous from a viewpoint of space saving. Creasing by using a laser beam is environmentally less favorable because smoke and odor are emitted during creasing.
A device that creases a sheet by pressing a creasing blade against the sheet can form a crease in a direction perpendicular to a sheet conveying direction in a relatively short period of time and easily. A required magnitude of pressing force for the creasing varies depending on a sheet type, a sheet size, or a sheet thickness. However, it is difficult to change the magnitude of the pressing force to be applied from the creasing blade for creasing. Accordingly, the pressing force is typically set to a highest pressing force among forces needed for sheets to be processed. This inevitably results in an increase in a driving load of the creasing blade. As the driving load increases, the device is upsized. Accordingly, loads placed on other parts are increased, making it necessary to increase strengths of the other parts. Furthermore, long-term use of the device can also cause a problem in reliability. Furthermore, when a large load is placed on a thin sheet that does not need a large load, an excessively deep crease is formed, resulting in a problem in quality.
There is a need that a crease can be formed on a sheet serving as a target for creasing with a minimum driving load.